Near-Edge X-ray Absorption Fine-Structure Study of Ion-Beam-Induced Phase Transformation in Gd{sub 2}(Tl{sub 1-y}Zr{sub y}){sub 2}O{sub 7}

The structural and electronic properties of Gd{sub 2}(Tl{sub 1-y}Zr{sub y}){sub 2}O{sub 7} (y = 0-1) pyrochlores following 2.0 MeV Au{sup 2+} ion-beam irradiation ({approx}5.0 x 10{sup 14} Au{sup 2+}/cm{sup 2}) have been investigated by Ti 2p and O 1s near-edge x-ray absorption fine structure (NEXAFS). The irradiation of Gd{sub 2}(Tl{sub 1-y}Zr{sub y}){sub 2}O{sub 7} leads to the phase transformation from the ordered pyrochlore structure (Fd3m) to the defect fluorite structure (Fm3m) regardless of Zr concentration. Irradiated Gd{sub 2}(Tl{sub 1-y}Zr{sub y}){sub 2}O{sub 7} with y {le} 0.5 are amorphous although significant short-range order is present. Contrasting to this behavior, compositions with y {ge} 0.75 retain crystallinity in the defect fluorite structure following irradiation. The local structures of Zr{sup 4+} in the irradiated Gd{sub 2}(Tl{sub 1-y}Zr{sub y}){sub 2}O{sub 7} with y {ge} 0.75 determined by NEXAFS are the same as in cubic fluorite-structured yttria-stabilized zirconia (Y-ZrO{sub 2}), thereby providing conclusive evidence for the phase transformation. The TiO{sub 6} octahedra present in Gd{sub 2}(Tl{sub 1-y}Zr{sub y}){sub 2}O{sub 7} are completely modified by ion-beam irradiation to TiO{sub x} polyhedra, and the Ti coordination is increased to eight with longer Ti-O bond distances. The similarity between cation sites and the degree of disorder in Gd{submore » 2}Zr{sub 2}O{sub 7} facilitate the rearrangement and relaxation of Gd, Zr, and O ions/defects. This inhibits amorphization during the ion-beam induced phase transition to radiation-resistant defect fluorite structure contrasting to the ordered Gd{sub 2}Ti{sub 2}O{sub 7}. Thus, during the ion-beam induced phase transition, the Gd{sub 2}(Tl{sub 1-y}Zr{sub y}){sub 2}O{sub 7} with y {le} 0.5 becomes amorphous whereas compositions with y {ge} 0.75 retain crystalline structure.« less